How can anyone kill the soul?

In the 21st verse of chapter 2 of Bhagavad Gita for happiness, Lord Krishna tells Arjuna that any human being who is aware that the soul is not destructible and eternal cannot kill anyone or ask anyone to kill. When Krishna is asking Arjuna to continue with the battle, it is for supreme justice — it is not about violence at all. It is within the discipline of fo justice.